Published on Web 03/18/2003
Diarylurea-Linked Zinc Porphyrin Dimer as a Dual-Mode Artificial Receptor: Supramolecular Control of Complexation-Facilitated Photoinduced Electron Transfer Shigeyuki Yagi,* Masayuki Ezoe, Isamu Yonekura, Toru Takagishi, and Hiroyuki Nakazumi Department of Applied Materials Science, Graduate School of Engineering, Osaka Prefecture UniVersity, 1-1 Gakuen-cho, Sakai, Osaka 599-8531, Japan Received November 26, 2002; E-mail:
[email protected] Well-defined electron donor-acceptor arrays in the photosynthetic reaction center ensure highly efficient photoinduced electron transfer (PET) and long-lived charge separation.1 Such natural photochemical functions have been receiving much attention in terms of the development of molecular-scale photonic devices aimed at light-energy conversion,2 and, thus, the construction of noncovalent assemblies of those components with photochemical and electrochemical properties suitable for PET has been one of the important subjects in supramolecular chemistry.3 From this viewpoint, porphyrin-based artificial receptors for redox active substrates have been widely studied because of the structural analogy to the photosynthetic pigments, where recognition of the acceptors such as quinone,3b,c,4 viologen,5 and fullerene6 has been intensively investigated. In the next stage, it is a challenging theme to devise molecular photonic systems with on-off controllable triggers, for example, the control of PET by supramolecular regulatory stimuli. Here we report the complexation-facilitated PET system consisting of a novel diarylurea-linked zinc porphyrin dimer 1 and a viologen derivative (hexyl viologen perchlorate, HV). We also report the inhibitory control of the PET using an alternative substrate.
Figure 1. (a) Electronic absorption spectra of 1-3, and (b) absorption spectral changes of 1 upon addition of increasing amounts of HV (0, 0.30, 0.70, 0.90, 1.2, 1.5, 2.5, and 5.0 µM; [1] ) 1.5 µM) in CHCl3/DMSO (10/ 1, v/v) at 293 K. Table 1. Binding Constants of 1-3 for HV As Determined by Electronic Absorption (Kabs) and Fluorescence (Kem) Spectroscopy
a
We previously reported the construction of cofacial porphyrin arrays using a diarylurea linkage (e.g., the single-linked dimer 2).7 The double-linked dimer 1 was synthesized from 5,10-bis(3-amino4-methylphenyl)-15,20-bis(4-methylphenyl)porphyrin in a manner similar to the preparation of 2. In Figure 1a are shown electronic absorption spectra of 1-3 in the Soret region. The absorption band of 2 exhibits a blue shift by 3 nm as compared with that of 3, the weak excitonic interaction between the porphyrin chromophores indicating that a cofacial conformer of 2 is predominantly formed. 4068
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compound
Kabs/M-1 a
Kem/M-1 a
1 2 3
546 000b 4300b 1100b
>1 000 000 3890b 2300b
In CHCl3/DMSO (10/1, v/v) at 293 K. b Errors are within 10%.
The shoulder in the monomer absorption region implies that the excitonic interaction is partially canceled due to the structural flexibility. On the other hand, the dimer 1 shows the narrow blueshifted band, and, thus, bridging two porphyrin moieties by two diarylurea linkers yielded a tightly fixed cofacial structure. The compounds 1-3 exhibited different binding abilities for HV. Adding increasing amounts of HV to solutions of 1-3 led to red shifts of the Soret absorption accompanying saturation behaviors with an isosbestic point due to the receptor-substrate complexation (Figure 1b). The stoichiometry for each complex was confirmed as 1:1 by either the molar ratio method or Job’s analysis.8 As summarized in Table 1, the binding constant Kabs of 1‚HV was much larger than those of 2‚HV and 3‚HV.9 In the 1H NMR spectra of the 2‚HV system, the signals assigned to the HV’s protons Ha, Hb, and Hc exhibited large complexation-induced shifts to the upfield due to ring-current anisotropy from the porphyrin π-planes, indicating that HV was bound to the cleft consisting of the two porphyrin moieties.10 Taking it into consideration that ZnTTolP11 did not form a complex with HV and that the increase of the diarylurea unit from 2 to 1 led to the increase of affinity to HV, charge-dipole interactions between HV and the two carbonyl groups in 1 directed into the inner of the cleft play an important role in the formation of the stable complex,12 in addition to the electrostatic interaction between HV and well-organized porphyrin moieties. The binding of HV was accompanied by fluorescence quenching of the porphyrin receptors. The fluorescence emission spectral 10.1021/ja0294717 CCC: $25.00 © 2003 American Chemical Society
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Figure 2. (a) Fluorescence emission spectra of 1 (1.5 µM) in the presence of varying concentrations of HV (a-i; 0, 0.30, 0.60, 0.90, 1.2, 1.5, 2.5, 3.0, 6.0 µM). (b) The fluorescence quenching profiles for 1‚HV, 2‚HV, and 3‚HV systems monitored at 608 nm. The emission spectra were obtained in CHCl3/DMSO (10/1, v/v) at 293 K. λex ) 562 nm.
cence emission was suppressed by complexation with HV, the addition of DABCO led to recovery of the emission, indicating DABCO competes with HV to kick it away from the cleft of 1 (Scheme 1).14 Indeed, in the 1H NMR spectra, the HV’s signals returned to their original positions upon addition of DABCO, whereas DABCO’s signal appeared at -4.76 ppm. In summary, we developed a novel dual-mode porphyrinic receptor 1 that binds HV and DABCO in different manners. The PET from 1 to HV was facilitated by complexation and suppressed by substrate exchange with DABCO, and, thus, the supramolecular control of the PET was achieved. Acknowledgment. This work was supported by a grant from the Iketani Science and Technology Foundation. Supporting Information Available: Synthetic procedure for 1 (PDF). This material is available free of charge via the Internet at http:// pubs.acs.org. References
Figure 3. Fluorescence emission spectra of (a) 1, (b) 1 + HV (3.0 µM), (c) 1 + HV (3.0 µM) + DABCO (15 µM), and (d) 1 + DABCO (15 µM) in CHCl3/DMSO (10/1, v/v) at 293 K. [1] ) 1.5 µM. λex ) 562 nm. Scheme 1
changes of 1 upon addition of HV are shown in Figure 2. As the concentration of HV increased, the emission intensity of 1 was reduced due to PET from the porphyrin to HV (Figure 2a).13 The spectral changes finally reached a plateau, indicating that the fluorescence quenching was induced by the complexation (Figure 2b). Although 2 and 3 also exhibited fluorescence quenching upon addition of HV, the quenching efficiency varied in the receptors. The binding constant Kem for each receptor obtained from the fluorescence spectral changes validly corresponded to Kabs (Table 1), and, therefore, the PET was exclusively facilitated by the complexation. It is interesting that the increase in magnitude of the binding constant led to the increase of the fluorescence quenching efficiency. Although the details are not clear at this point, the well-defined structure of 1 affording tight fixation of HV should give rise to correct donor-acceptor orientation. The diarylurea linker also affords enough space between two porphyrin rings to bind 1,4-diazabicyclo[2.2.2]octane (DABCO) through two Zn-N coordination interactions.7 Thus, the inhibitory control of the PET was examined by addition of DABCO to a solution containing 1 and HV (Figure 3). Although the fluores-
(1) (a) Deisenhofer, J.; Michel, H. Angew. Chem., Int. Ed. Engl. 1989, 28, 829. (b) Huber, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 848. (2) (a) Balzani, V.; Scandola, F. Supramolecular Photochemistry; Ellis Horwood: London, 1991. (b) Wasielewski, M. R. Chem. ReV. 1992, 92, 435. (c) Osuka, A.; Nakajima, S.; Maruyama, K.; Mataga, N.; Asahi, T.; Yamazaki, I.; Nishimura, Y.; Ohno, T.; Nozaki, K. J. Am. Chem. Soc. 1993, 115, 4577. (d) Kurreck, H.; Huber, M. Angew. Chem., Int. Ed. Engl. 1995, 34, 849. (e) Wagner, R. W.; Lindsey, J. S.; Seth, J.; Palaniappan, V.; Bocian, D. F. J. Am. Chem. Soc. 1996, 118, 3996. (f) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A. L.; Gust, D.; Moore, T. A. Nature 1997, 385, 239. (g) Uosaki, K.; Kondo, T.; Zhang, X.-Q.; Yanagida, M. J. Am. Chem. Soc. 1997, 119, 8367. (h) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (i) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (3) (a) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1993, 115, 10418. (b) Kuroda, Y.; Ito, M.; Sera, T.; Ogoshi, H. J. Am. Chem. Soc. 1993, 115, 7003. (c) Hunter, C. A.; Shannon, R. J. Chem. Commun. 1996, 1361. (d) Hayashi, T.; Ogoshi, H. Chem. Soc. ReV. 1997, 26, 355. (e) Ward, M. D. Chem. Soc. ReV. 1997, 26, 365. (f) Hu, Y.-Z.; Takashima, H.; Tsukiji, S.; Shinkai, S.; Nagamune, T.; Oishi, S.; Hamachi, I. Chem.Eur. J. 2000, 6, 1907. (g) Ikeda, A.; Hatano, T.; Shinkai, S.; Akiyama, T.; Yamada, S. J. Am. Chem. Soc. 2001, 123, 4855. (4) (a) Aoyama, Y.; Asakawa, M.; Matsui, Y.; Ogoshi, T. J. Am. Chem. Soc. 1991, 113, 6233. (b) Hayashi, T.; Miyahara, T.; Koide, N.; Kato, Y.; Matsuda, H.; Ogoshi, H. J. Am. Chem. Soc. 1997, 119, 7281. (5) (a) Gunter, M. J.; Hockless, D. C. R.; Johnston, M. R.; Skelton, B. W.; White, A. H. J. Am. Chem. Soc. 1994, 116, 4810. (b) Gunter, M. J.; Jeynes, T. P.; Johnston, M. R.; Turner, P.; Chen, Z. J. Chem. Soc., Perkin Trans. 1 1998, 1945. (6) (a) Tashiro, K.; Aida, T.; Zheng, J.-Y.; Kinbara, K.; Saigo, K.; Sakamoto, S.; Yamaguchi, K. J. Am. Chem. Soc. 1999, 121, 9477. (b) Kubo, Y.; Sugasaki, A.; Ikeda, M.; Sugiyasu, K.; Sonoda, K.; Ikeda, A.; Takeuchi, M.; Shinkai, S. Org. Lett. 2002, 4, 925. (c) Guldi, D. M.; Ros, T. D.; Braiuca, P.; Prato, M.; Alessio, E. J. Mater. Chem. 2002, 7, 2001. (7) Yagi, S.; Yonekura, I.; Awakura, M.; Ezoe, M.; Takagishi, T. Chem. Commun. 2001, 557. (8) The stoichiometry was determined for 1‚HV by the molar ratio method using fluorescence emission spectroscopy and for 2‚HV and 3‚HV by Job’s method using 1H NMR. (9) The binding constants were determined by the least-squares analyses of the absorbance changes according to a 1:1 complexation model. (10) The complexation-induced shift of the protons Ha, Hb, and Hc is 0.92, 1.34, and 1.64 ppm upfield, respectively, in the 1:1 mixture of 2 and HV ([2] ) [HV] ) 1.0 mM in CDCl3/DMSO-d6 (10/1, v/v) at 293 K). In 1‚HV, the receptor-substrate association and dissociation was slow enough to afford broadening signals of HV’s protons, although the upfield shifts of Ha-Hc were observed. (11) ZnTTolP: [5,10,15,20-tetrakis(p-tolyl)porphyrinato]zinc(II). (12) Ong, W.; Gomez-Kaifer, M.; Kaifer, A. E. Org. Lett. 2002, 4, 1791. (13) PET from porphyrins to viologens have been well known: (a) Akins, D. L.; Zhu, H.-R.; Guo, C. J. Phys. Chem. 1993, 97, 8681. (b) Hayashi, T.; Takimura, T.; Ogoshi, H. J. Am. Chem. Soc. 1995, 117, 11606. (c) Kaganer, E.; Joselevich, E.; Willner, I.; Chen, Z.; Gunter, M. J.; Gayness, T. P.; Johnson, M. R. J. Phys. Chem. B 1998, 102, 1159. (14) The Kabs for 1‚DABCO was determined as 695 000 M-1 (error 8.4%).
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